This invention pertains to an improvement in artifact rejection methods for noninvasive blood-pressure measurement, and more particularly, to a unique method of predicting, from acquired data, subsequent, expected data values.
Examples of artifact rejection methods for noninvasive blood-pressure measurement in the closest prior art are disclosed in U.S. Pat. No. 4,367,751 to Link, et al. and U.S. Pat. No. 4,543,962 to Medero, et al.
In the typical practice of an oscillometric, noninvasive blood-pressure measurement method with a person, a counterpressure-producing cuff is wrapped around the person's upper arm, with the cuff then inflated to a counterpressure above systolic pressure to occlude an artery (blood vessel) in the arm. Thereafter cuff counterpressure is progressively reduced in a stepped fashion from this suprasystolic pressure to an ending counterpressure where the cuff is substantially deflated. During progressive reduction of cuff counterpressure, the artery opens progressively from an occluded state to an unoccluded state.
During the change from the occluded state to the unoccluded state, the artery begins to product detectable pulsations against the cuff, and the waveforms of these pulsations are monitorable to produce graphic illustrations of blood-pressure parameters. As is well-known to those skilled in the art in handling blood-pressure measurements, the pulsations just referred to increase in amplitude toward a maximum as cuff counterpressure decreases below systolic pressure, and then decrease in amplitude. By categorizing these pulsations relative to their occurrences in time and to their respective amplitudes, desired blood-pressure parameters are determined.
In all methods of acquiring usable blood-pressure data, it is important to detect, and reject, as faithfully as possible, pressure "artifacts" which are not induced by blood-pressure pulsations. Artifacts occur, for example, where a subject moves, changes muscle tension, etc.
The method of the present invention significantly addresses this key issue.
An important object of the invention, therefore, is to provide for artifact rejection in a unique way which ensures that accepted pressure waveforms truly are blood-pressure induced.
Another object is to decrease the number of pressure waveforms that are monitored at each cuff counterpressure level, thereby to decrease the overall time period of a measuring cycle, thus to maximize efficiency.
By way of introduction, applicants' novel artifact rejection method is disclosed as being practiced in conjunction with an "impulse-based method" of non-invasive blood-pressure measurement (NIBP). This "impulse-based method" of NIBP is fully disclosed in a prior-filed patent application, Ser. No. 07/198,468, filed May 25, 1988, by Craig H. Nelson, Thomas J. Dorsett and Charles L. Davis, for "METHOD FOR NONINVASIVE BLOOD-PRESSURE MEASUREMENT BY EVALUATION OF WAVEFORM-SPECIFIC AREA DATA".
It should be understood that while the present invention is being disclosed in the measurement setting described in the above-referenced patent application, in which setting the invention has been shown to offer particular utility, the invention can also be used in a variety of blood pressure measurement systems which acquire blood-pressure data in different ways. For example, a number of systems are currently available wherein blood pressure parameters are derived from monitoring the amplitudes rather than the impulses characterizing blood pressure induced waveforms.
To deal with the issue of false "artifact" data according to the method of, the present invention, a rejection technique is employed which verifies that developed blood pressure data values are proper values by employing what might be thought of as "pulse-prediction" and "pulse-adjustment".
The theory applied by applicants in carrying out their unique artifact rejection method is based on what is known in the art, and what will be described as, "Kalman Filter theory." Applicants used an algorithm based on the "Kalman Filter" to provide for what will be described as "pulse-prediction." Further, applicants applied what is known in the art as "fixed lag smoothing" to provide for what will be described as "pulse-adjustment."
Applying this theory, the present invention is employed after development of waveform data values for a predetermined number of cuff counterpressure levels at the beginning of a measuring cycle. Specifically, the technique is used to predict a next, expected-to-be-encountered data value for a next, lower cuff counterpressure level. Employing prediction for successive, next, lower cuff counterpressure levels provides a simple and accurate method of artifact rejection which also substantially decreases the number of pressure waveforms required to be monitored at a given cuff counterpressure level. Therefore, if a next, developed waveform data value for a measured waveform is within experimentally set upper and lower bounds of its corresponding predicted value, the measured value is accepted as being blood-pressure induced.
This important feature of ongoing next-to-be-expected data value prediction, significantly enhances the likelihood that a false data pulse will be rejected as an artifact.
Using the prediction technique just described for subsequent cuff counterpressure levels, it will generally be necessary to monitor only one pressure waveform at a given cuff counterpressure level. As will be explained, if the first pressure waveform which is monitored does not have a data value that is within the upper and lower bounds of its corresponding predicted value, subsequent waveforms will be monitored until one is found which does meet the boundary conditions. This situation, of "looking" for successive, subsequent "boundary-meeting" waveforms, continues only for a predetermined ultimate time interval, after which, if no proper waveform is found, the method of the invention aborts the measurement cycle.
In addition, and further in accordance with special features of the invention, the proposed rejection technique is employed further to perform a "pulse-data adjustment". Thus, the technique adjusts previously encountered (and stored) waveform data values based on the difference between a measured waveform data value and a corresponding predicted waveform data value for a given cuff counterpressure level. This is referred to herein as a "smoothing" technique. By adjusting previously stored values, smoothing provides further ensurance of the accuracy of ultimately derived, desired blood-pressure parameters.
These and other objects and advantages which are attained by the invention will become more fully apparent as the description that now follows is read in conjunction with the accompanying drawings and computer program flow charts.